The evolution of high speed packet access (HSPA) is being considered for higher throughput and lower latencies. Due to the increase of data services, in particular internet services such as web browsing, where high data rates are requested for short periods of time, the third generation partnership project (3GPP) Release 99 (R99) mechanism of transitioning wireless transmit/receive units (WTRUs) in CELL_FACH to CELL_DCH requires significant network resources and adds latency to the services. To support these types of services in CELL_FACH, it has been proposed that the WTRUs are allowed to use the enhanced dedicated channel (E-DCH) with shared resources without transitioning to CELL_DCH, which is referred to as “enhanced random access channel (E-RACH) access” or “E-DCH in CELL_FACH state and Idle mode”.
An E-RACH access is a combination of a random access channel (RACH) preamble transmission phase and an E-DCH transmission phase. FIG. 1 shows an E-RACH access procedure. The RACH preamble transmission phase uses a subset of R99 RACH signatures that a Node B has designated or broadcasted for use in E-RACH. The reception of a preamble by the Node B is acknowledged in an acquisition indication channel (AICH), which also assigns a WTRU with an index for a shared E-DCH resource to use. The shared E-DCH resources are pre-designated by the Node B for use in an E-RACH access in CELL_FACH. For all shared E-DCH resources, the parameters are provided to the WTRU during initial setup or broadcast to WTRUs in the cell by the Node B. Each E-DCH resource is associated with an index which is transmitted as part of the acknowledgement for the E-RACH access, or using some other signaling mechanism.
Once the WTRU receives the index value, all configuration parameters related to the assigned shared E-DCH resource are known and the WTRU does not need to communicate with the Node B in the same way as in R99. Indeed, in E-RACH, the E-DCH is used for the message transmission instead of the regular R99 10 or 20 ms physical random access channel (PRACH) message part.
The E-RACH access eliminates the overhead associated with the conventional CELL_FACH to CELL_DCH transition. The shared E-DCH resource is released upon completion of data transfer and the WTRU remains in CELL_FACH so that other WTRUs could use the shared E-RACH resources. Thus, a significant reduction in transition latency is achieved and it eliminates transition back to CELL_FACH with re-initialization when CELL_DCH is terminated. The WTRU may request permanent transition to CELL_DCH directly from E-RACH access.
The conventional RACH access starts with preamble transmission of one randomly selected PRACH signature out of a set of up to 16 signatures, with a preconfigured initial power level. If no response is received on the associated AICH from the Node B, the WTRU selects the next available access slot, increases power by a predefined amount and transmits a new randomly selected signature from the set of available signatures. If the maximum number of preamble transmissions is exceeded or negative acknowledgement (NACK) is received, the WTRU exits the PRACH access procedure and reports it to a higher layer, (i.e. medium access control (MAC)).
If a positive acknowledgement (ACK) response is received from the Node B, the WTRU transmits a RACH message three or four uplink access slots after the uplink access slot of the last transmitted RACH preamble. FIG. 2 shows timing relationship between RACH access slots and AICH access slots. The RACH access slot precedes the corresponding AICH access slot by τp-a. For instance, if the WTRU transmits a preamble on PRACH access slot #2, the WTRU may get an ACK response on AICH access slot #2, and the WTRU may begin transmission of the RACH message on PRACH access slot #5 or #6, depending on the WTRU capabilities.
3GPP Release 8 (R8) E-RACH access during CELL_FACH starts with RACH preamble transmission followed by shared E-DCH transmission when an E-DCH resource is assigned by the Node B as shown in FIG. 1. A NACK or no response from the Node B requires the WTRU to transmit again in the next available access slot until the maximum number of attempts has been exhausted. The Node B responds to the RACH preamble via an AICH as in R99. The timing of the start of the E-DCH transmission has been agreed as a fixed time offset relative to the fractional dedicated physical channel (F-DPCH) frame timing (same as regular E-DCH). The F-DPCH timing offset, expressed by the variable τF-DPCH, p, is set by the network and may be different for different F-DPCHs, but the offset from the P-CCPCH frame timing is always a multiple of 256 chips. FIG. 3 shows the radio frame timing and access slot timing of downlink physical channels. FIG. 4 shows the downlink and uplink timing relationship between primary common control physical channel (P-CCPCH), AICH, F-DPCH, dedicated physical control channel (DPCCH) and E-DCH.
One of the issues associated to the use of E-DCH in CELL_FACH state and Idle mode resides in determining the F-DPCH frame timing. In the conventional systems, the F-DPCH frame timing is signaled explicitly by the network when the WTRU is transitioned to CELL_DCH. The F-DPCH frame dictates the beginning of the DPCCH preamble transmission, which essentially determines the start of the uplink scrambling code sequence. Since it is difficult to initialize the scrambling code in the middle of a frame, it is typically necessary for the WTRU to start uplink transmission after crossing the frame boundary at least once.
Fixing the F-DPCH frame timing to the P-CCPCH, as it is currently done in CELL_DCH, may cause difficulties with E-DCH in CELL_FACH. Indeed, it may lead to a possibility of power control update being delayed by as much as a full frame (10 ms). To illustrate, consider an E-RACH preamble transmitted in an access slot near the end of an uplink E-DCH frame, as would be defined by the E-DCH shared resource F-DPCH frame timing (with respect to the fixed P-CCPCH). Assuming an ACK is transmitted over the AICH, this ACK response will be received at the end of the uplink E-DCH frame or at the beginning of the next uplink E-DCH frame so that the WTRU may not have an opportunity to transmit (due to the need to initialize the scrambling code) until the beginning of the next E-DCH frame. This will result in a long delay before the power control loop can be established, essentially making the first power control update nearly a full frame, (i.e., 10 ms), after the last RACH preamble transmission. This may cause power control loop stabilization problem. It may also result in additional latency for sending the RACH message part. Thus, for a given F-DPCH offset relative to the P-CCPCH, some access slots will be more advantageous than others for E-RACH access, and some access slots may not be preferable due to power control update latency.